In communication systems where voice or data is transmitted from one unit to another by a radio frequency signal modulated by digital signals representative of information content of the voice or data, and wherein the transmitter and receiver may be operating on the same or different frequencies at any time prior to acquisition, a system is required to acknowledge acquisition and timing signal recovery to insure that the transmitter in a first communications unit of the system and a receiver in a second communications unit of the system are operating on the same frequency at the same time, even in frequency hopping mode.
One or more frequency channels are used for acquisition and timing recovery. These channels are scanned in a standby mode to detect a signal requesting communication. If a single channel is used for acquisition and timing recovery, jamming interfering signals or frequency selective fading might render that single channel unusable and therefore prevent communication until the acquisition and timing recovery channel becomes available. Further, the use of the single channel for acquisition and timing recovery may result in more than one receiver attempting to acquire the link and communicate with a transmitting station resulting in ambiguity.
Further, in either single channel or multichannel acquisition acknowledgment systems, the originating unit attempts to pass information to a remote unit and requests an acknowledgment in return. If the remote unit is acquired and transmits an acknowledgment, the transmitting station is ready to proceed with communication of information, but the remote station has not received any signal from the transmitting station indicating that the acquisition acknowledgment has been received by the transmitting station. This could result in a never ending series of attempts at acquisition acknowledgment which would effectively prevent communication between the two stations.
If the system is in frequency hopping mode, the problem of acquisition acknowledgment is made more severe due to the fact that a transmitting station might change frequency before an acknowledgment is received from a remote station and thus have its receiver on a different frequency when an acknowledgment is transmitted from the remote station to the original transmitting station. Thus, it is very important in such communication systems that each station be in synchronism with respect to beginning the frequency hopping sequence.